1. Field of the Invention
This invention relates to a corrosion inhibiting pigment.
2. Description of the Prior Art
The use of crystallized zinc phosphate, either as the tetrahydrate or as dihydrate, as a corrosion inhibiting pigment has been known for some time. It is offered on the market in the form of finely divided white powder, constituted by microscopic crystals having the form of hexagonal slabs which may have a major diagonal dimension of up to 25 .mu.m and a thickness of up to 4 .mu.m. This product is used in the preparation of corrosion inhibiting primers forming part of protective coatings for metals. The limitations of this pigment are also known, since it does not give good results in the accelerated tests which have to be carried out prior to the approval of any protective coating system. To act, it needs a slow hydrolysis which does not have time to occur during the time the coating is exposed in a salt spray chamber (ASTM B-117). This means that the product is initially rejected in the accelerated prior tests in spite of the fact that it gives good results (effective protection) in long term weather exposure in primers. Many attempts have been made to improve this immediate effectivity by modifying the chemical composition with additions of other substances which, although they accelerate the immediate protective result, at long term suffer from the drawbacks of blistering and loss of adhesion of the coating.
It is the finding of this invention that the above mentioned drawbacks are overcome by using a pigment which, while being constituted by zinc phosphate dihydrate or tetrahydrate or a mixture of both, instead of consisting of slabs like zinc phosphate pigments of the prior art, consists of spheroidally shaped particles, each of which is made up of smaller zinc phosphate lamellar microcrystals which are generally disposed radially to form a pigment particle of spheroidal shape and rough surface, said rough surface consisting of the edges of the radially disposed zinc phosphate microcrystals. These spheroids have dimensions comprised between 0.5 and 6 microns, with a mean size between 1.5 and 2.5 microns, and the radially disposed microcrystals which constitute the particle have in turn a maximum dimension between 0.05 and 0.5 microns.
Although attempts have been made to improve the performance of prior art zinc phosphate pigments by reducing their particle size, no improvement was achieved. It is thus a finding of this invention that it is the shape of the zinc phosphate pigment particles which is relevant towards improving the performance of the chemical in accelerated tests, and specifically that clear advances are shown with respect to prior art zinc phosphate in corrosion inhibiting effectivity, adherence and (lack of) blistering of the protective films, as evidenced by salt spray and distilled water immersion tests at 50.degree. C., when zinc phosphate pigments of spheroidal particle shape are used instead of the prior art slab shaped pigments.
The process for manufacturing said pigments comprises the following steps:
A finely dispersed slurry of zinc oxide is prepared, by slowly adding zinc oxide powder to a tank filled with water, with vigorous stirring in order to prevent the formation of lumps; the temperature may be between 20.degree. and 30.degree. C., and the final concentration about 25% by weight. This slurry is allowed to rest for 17 hours; afterwards it is circulated through a powerful mechanical disperser with a speed of 27,000 r.p.m., such as the Polytron (Switzerland) or Ultraturrax-IKA (W. Germany) types, which are both high frequency kinematic machines.
With the manufacture of the pigment of this invention being the reaction between a slurried solid (ZnO) and a liquid (aqueous solution of phosphoric acid and ammonium salt), care must be taken to make sure that the particles of the slurried solid are small enough as to make a large reaction surface available. Specifically, care must be taken that the surface of the zinc oxide particles (normally produced by combustion of the vaporized molten zinc) reacts with the water to give the hydroxide. The extent to which this reaction takes place in terms of the total mass of zinc oxide involved does not really matter, it is just required that the surface of the particles should have reacted with water, in order to avoid the need of excess phosphoric acid, which would result in an undesirable growth of the zinc phosphate particles. This hydration of the surface of the zinc oxide is facilitated when particles are made small enough, and this may be monitored by a change in the pH of the slurry from 6.2-6.6 for an "unactivated" (unreacted) slurry to 7.5-8.0 for an "activated" (surface hydrated) zinc oxide slurry.
In another tank, phosphoric acid of 85% purity is diluted with water, and an organic or inorganic ammonium salt is added to the solution. The purpose of adding the ammonium salt is to form a zinc-ammonia complex, which is believed to interfere with the zinc phosphate microcrystals nucleation and growth processes, resulting in spheroidal particles. It is a fact that it is the presence of plenty of ammonium ion that results in the unique morphology of zinc phosphate particles, which is a characteristic of this invention. It is not a limit to this invention that the ammonium salt be added to one of the two fluids which are mixed for reaction, namely the phosphoric acid containing solution as in Example 1. It may be also added to the zinc oxide slurry, or alternatively it may also be divided between both reacting fluids, in any ratio as long as the total required amount is used; in every case the desired morphology is obtained just the same. Nevertheless, it is practical to add the ammonium salt to the phosphoric acid solution in order to avoid a substantial thickening of the activated zinc oxide slurry. The amount of ammonium salt, expressed as ammonium chloride, needed to obtain the desired morphology may lie between 20 and 60% of the weight of ZnO used.
Thus, preferably, the activated zinc oxide slurry and the ammonium salt containing phosphoric acid solution are simultaneously poured into a small reactor with continuous overflow to a bigger tank, the reactor being stirred with a strong shear, for which dispersers with speeds between 22,000 and 27,000 r.p.m. are suitable. Throughout the pouring of the fluids, excess zinc oxide over the stoichiometric amount required for zinc phosphate formation is used; thus, at the end of the process, a final amount of phosphoric acid solution is poured alone into the small reactor, to eventually react with zinc oxide inside the bigger tank.
After 40 minutes of moderate stirring in the bigger tank, the formed zinc phosphate may be observed under a microscope as spheroidal particles covered by small microcrystals. The precipitate is then filtered and washed until foreign soluble salts are removed, and is subsequently dried and ground to give a pigment constituted essentially by zinc phosphate dihydrate and/or tetrahydrate (depending on the temperature at which it has been dried), with from 0.5 up to 6% excess of zinc oxide over the stoichiometric amount, formed by rough-surfaced spheroids having a maximum dimension lying between 0.5 and 5 microns and a mean size of 1.5 to 2.5 microns.